In a mobile communication system, it is common that a terminal (to be referred to as “user apparatus UE” hereinafter) and a base station BS performs communication so that communication is performed between user apparatuses UE. However, in recent years, various techniques are considered for performing direct communication between user apparatuses UE.
When performing communication between user apparatuses UE, it is necessary that a user apparatus UE discovers another neighboring user apparatus UE. As a method for discovering a user apparatus UE, there is a method in which each user apparatus UE transmits (broadcasts) a discovery signal including its own ID (identification information).
FIG. 1 is a diagram showing an example of a resource for transmitting a discovery signal. In the example of FIG. 1, it is defined that a discovery period, for performing discovery (and being discovered) of a user apparatus UE by transmitting and receiving a discovery signal, comes periodically. In each discovery period, a predetermined number of resources (each being a time-frequency resource, which is to be referred to as “discovery resource”) for transmitting (and receiving) a discovery signal are defined. Each user apparatus UE transmits a discovery signal by using a discovery resource in the discovery period.
For example, a user apparatus UE1 of FIG. 2 transmits a discovery signal using a discovery resource indicated by UE1 of FIG. 1, a user apparatus UE2 of FIG. 2 transmits a discovery signal using a discovery resource indicated by UE2 of FIG. 1.
A user apparatus UE3 in FIG. 2 discovers the user apparatus UE1 by receiving a discovery signal transmitted by the user apparatus UE1, and a user apparatus UE4 discovers the user apparatus UE2 by receiving a discovery signal transmitted by the user apparatus UE2. Communication of the discovery signal is half-duplexing. Therefore, transmission and reception cannot be performed simultaneously. For example, in FIG. 2, in a case where the user apparatus UE1 transmits a discovery signal using a discovery resource of UE1 shown in FIG. 1, if the user apparatus UE3 transmits a discovery signal using a discovery resource indicated by A of FIG. 1, the user apparatus UE1 and the user apparatus UE3 cannot receive a discovery signal of the other party, and does not discover the other party. There is a patent document 1 as a prior art document on terminal-to-terminal communication.